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This invention relates to inkjet printing technology, and laser-scanning technology where a laser is used to transfer image data.
Currently, two commonly used technologies for imaging are laser (also referred to herein as xe2x80x9celectrophotographicxe2x80x9d) systems, and ink jet systems. In both of these systems, digital image data, produced by a computer, or the like, is transferred to the printer, which renders this data as a visible image upon a media. In most computer and printer systems, the image data for the printer is digital data which is stored in computer memory. This is the case for inkjet and laser printers, including both color and monochrome. The data is stored in a matrix or xe2x80x9crasterxe2x80x9d which identifies the location and color of each pixel which comprises the overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the technology to which the invention described below is applicable to either photocopiers or printers. Recent technology has removed this distinction, such that a single printing apparatus can be used either as a copier or as a printer for computer files. These apparatus have been known as multifunction printers (xe2x80x9cMFPs)xe2x80x9d, a term indicating the ability to act as a photocopier, a printer, or a facsimile machine. Accordingly, the expression xe2x80x9cprinterxe2x80x9d should not be considered as limiting to a device for printing a file from a computer, but should also include a photocopier capable of printing a digitized image of an original document. xe2x80x9cOriginal documentsxe2x80x9d include not only already digitized documents such as text and image files, but photographs and other images, including hybrid text-image documents, which are scanned and digitized into raster data.
In any event, the image to be printed onto tangible media is stored as a digital image file. The digital image data is then used to drive a printing element to create an image. The raster image data file is essentially organized into a two dimensional matrix, that is translated by the printer into an image on the media. The image comprises a number of lines with each line comprising a number of discrete dots or pixels across the line. Each pixel in the image is assigned a binary value in the data file relating information pertaining to its color and potentially other attributes, such as density. The combination of lines and pixels makes up the resultant image.
As described the raster data is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each pixel in the line. A computer processor reads the raster image data line by line, and actuates the printer. For laser printers, this involves actuation of a laser that scans a photosensitive surface to selectively expose a pixel on the surface, based on the presence or absence of coloration, and the degree of coloration for the pixel. Typical pixel densities for images are in the range of 300 to 1200 pixels per inch, in each direction. For inkjet printers, actuation of the printer involves selective actuation of an inkjet nozzle to form, based upon the presence of absence of coloration, pixels upon a media surface.
Scanning in Laser Printers
In laser printers, the method of transferring the digital raster data to a photoconductor via a laser, lasers or LEDs is known as the image scanning process or the scanning process. The scanning process is performed by a scanning portion or scanning section of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the developing process. The developing process is accomplished by the developer section of the printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning section of the printer, which transfers the raster data image to the photoconductor, as well as the developer section of the printer, which manages the transfer of the toner to the photoconductor.
In the scanning process, a laser is scanned from one edge of the photoconductor to the opposing edge and is selectively actuated or not actuated on a pixel-by-pixel basis to scan a line of the image onto the photoconductor. The photoconductor advances and the next line of the image is scanned by the laser onto the photoconductor. In a multiple laser printer, more than one laser can be actuated simultaneously so as to more quickly generate the complete image onto the photoconductor. The side-to-side scanning of each laser is traditionally accomplished using a dedicated multi-sided or faceted rotating mirror. Such a mirror will be known herein as a xe2x80x9cpolygonxe2x80x9d due to the polygonal shape of the mirror. The reflective surface of the mirrors is typically ground and polished aluminum. The laser beam impinges on one facet of the polygonal mirror and is reflected to a secondary or deflector mirror, which directs the laser beam to a unique, relative lineal position on the light sensitive surface of the photoconductor. By xe2x80x9crelativexe2x80x9d, it is understood that the photoconductor moves with respect to the linear position, but the position remains fixed in space. As the polygonal mirror rotates, the angle of incidence, and hence the angle of reflection, of the laser beam will vary. This causes the laser beam to be scanned across the photoconductor at the unique relative lineal position from a first edge to a second edge of the photoconductor. As the mirror rotates to an edge of the polygon between facets, the laser is essentially reset to the first edge of the photoconductor to begin scanning a new line onto the photoconductor. These mirrors tend to rotate at very high speeds, often in excess of 20,000 rpm.
Examples of laser scanning systems used in laser printers are disclosed in U.S. Pat. Nos. 5,691,759; 5,745,152; 5,760,817; 5,870,132; 5,920,336; 5,929,892; and 6,266,073 which are hereby incorporated by reference.
Inkjet Printheads
Most commercial inkjet printers use a moving or scanning printhead system wherein a printhead comprising ink nozzles is moved or scanned across the surface of a media. As the printhead moves over the surface, each ink nozzle is selectively activated to eject an inkjet or ink droplet to form a pixel on the media as the head passes over the surface.
To eject the droplet, ink is delivered under pressure to a printhead nozzle area. According to one method, the ink is heated causing a vapor bubble to form in a nozzle which then ejects the ink as a droplet. Droplets of repeatable velocity and volume are ejected from respective nozzles to effectively imprint characters and graphic markings onto a printout.
An inkjet printhead is formed by a substrate plus several layers defining multiple nozzle areas. The substrate and layer qualities and dimensions are selected to achieve desired thermodynamic and hydrodynamic conditions within each nozzle. Various patents teach aspects of printhead fabrication, including U.S. Pat. Nos. 4,513,298 (Scheu); 4,535,343 (Wright et al.); 4,794,410 (Taub et al.); 4,847,630 (Bhaskar et al.); 4,862,197 (Stoffel); and 4,894,664 (Tsung Pan), which are incorporated by reference.
Conventional inkjet printheads extend over a limited portion of a page-width and scan across the page. This contrasts with a page-wide-array (xe2x80x9cPWAxe2x80x9d) printhead that extends over an entire page-width (e.g., 8.5xe2x80x3, 11xe2x80x3, A4 width) and is fixed relative to the media path. The PWA printhead is formed on an elongated printbar and includes thousands of nozzles. The PWA printbar is generally oriented orthogonally to the paper path. During operation, the printbar and the PWA printhead are fixed while a page is fed adjacent to and moves under the printhead. The PWA printhead prints one or more lines at a time as the page moves relative to the printhead. This compares to the printing of multiple characters at a time as achieved by scanning-type printheads.
In a PWA inkjet printhead the printhead includes a flexible printed circuit (xe2x80x9cflex circuitxe2x80x9d) coupled to the printbar. Attached to the flex circuit are silicon substrates in which are formed nozzle chambers with firing resistors. The flex circuit with silicon substrates is adhesively attached to the printbar. The printbar includes recessed areas for receiving respective silicon substrates. Signal paths in the flex circuit carry signals to the firing resistors. An addressed firing resistor heats up ink in a corresponding nozzle chamber resulting in an ejection of an ink droplet.
The printhead of a PWA inkjet printer includes thousands of nozzles. For an 11-inch printhead printing at 600 dpi, there are at least 6600 nozzles along the printhead. Ink is delivered from a resident reservoir to a nozzle chamber of each nozzle. During operation, the printer element is fixed while a page is fed adjacent to the printhead by a media handling subsystem. When printing, a firing resistor within a nozzle chamber is activated so as to heat the ink therein and cause a vapor bubble to form. The vapor bubble then ejects the ink as a droplet. Droplets of repeatable velocity and volume are ejected from respective nozzles to effectively imprint characters and graphic markings onto a media sheet. The PWA printhead prints one or more lines at a time as the page moves relative to the printhead. Examples of PWA printer systems are disclosed in U.S. Pat. Nos. 5,589,865; 5,719,602; 5,734,394; 5,742,305; and 6,135,586 which are hereby incorporated by reference.
The PWA printhead contrasts with the moving or scanning printheads, where scanning type printheads scan across a page while the page is intermittently moved by a media handling subsystem. A PWA printer element is analogous to the moving printhead as both eject ink drops upon a media surface that has relative movement to the printhead. However, the PWA has substantially more nozzles and it is fixed in position. There is relative movement between the printhead and the media in both PWA and moving printhead systems, which accounts for some similarities in construction. However, a PWA printhead is fixed, and typically much larger that a moving printhead. A PWA printer element can include several thousand nozzles extending the length of a page-width, while that of a conventional moving printhead usually has between 100 and 300 nozzles extending a distance of approximately 0.15 to 0.50 inches.
One of the driving motivations for creating a page-wide-array printhead is to achieve faster printing speeds. In particular it is desirable that a PWA printhead run at a print speed approaching nozzle speed. Nozzle speed is the highest frequency at which a nozzle is capable of firing as limited by nozzle technology, which under current technologies approaches 1500 Hz for conventional inkjet printers, and up to 6000 to 8000 Hz for certain high resolution inkjet printers. Print speed in a PWA is directly related to the frequency at which nozzles are actually fired during a print operation. Print speed typically is less than the maximum nozzle speed due to limitations in data handling (i.e., data throughput) and media handling. With more nozzles the PWA printer element should print much faster than a smaller scanning printhead, but because of limitations, particular with data handling, the potential speed of PWA systems has not been reached. Conceivably, with faster data throughput, the printing speed could be faster than many laser printers. Given a 1000 Hz firing rate for the inkjet nozzles, which is well within the rate commonly achieved in current inkjet printers, the printing speed could be 13.8 inches second over the width of the page for a 600 dpi resolution. Basically, a PWA printhead should be able to print an entire page in approximately the same timeframe it takes a moving printhead to make one scan across a page. If the data handling for the many thousand of nozzles in a PDA can be achieved in the same time frame as the data handling for the relatively few nozzles in a conventional moving printhead, the potential speed of the PWA can be more closely realized.
A part of the data-handling problems in a PWA is to assure that pixel or dot data is available at each nozzle in a timely fashion. With thousands more nozzles than a conventional scanning printhead, the rapid data transfer to achieve such data throughput is a significant challenge. Directly connecting the raster data memory storage and processor in parallel fashion could conceivably achieve a rapid data transfer, but because of the high number of nozzles and the high number of separate conductors and connectors that this would require, such an approach is not practical. A solution to this problem is to reduce the number of conductors and use any of a number of multiplexing schemes, wherein the firing signals are processed and firing signals for several nozzles are sent serially over a common conductor. While these systems significantly reduce the number of conductors required for the data transfer and make PWA construction practical, the data processing involved and the inherently slower communication rate for serial, as compared to parallel communication, significantly slows the rate of data transfer. Thus a challenge that has not yet been met is to increase the rate of data transfer for the thousands of the nozzles within the space constraints of a print head.
An aspect of the present invention is an imaging apparatus comprising a media transport for transporting media through a print zone, a page-wide-array inkjet printhead, and a photodetector array associated with the PWA printhead that is adapted to receive data from a laser scanner. The media transport is any suitable system known in the art for use with the PWA inkjet system, such that a PWA printhead is disposed with respect to the media to image the media as it is transported through the print zone. The PWA printhead comprises a plurality of the inkjet nozzles activated by an electrical pulse. When activated the nozzles create alphanumeric text, graphics and/or images by selectively applying ink drops to a pixel grid on the media surface as it passes under the nozzle. The photodetector array is associated with the PWA printhead and comprises a plurality of photodetectors with each photodetector of the photodetector array electrically connected to one of the nozzles. Upon light activation, the photodetector generates the electrical pulse to activate the nozzle.
The laser scanner is so disposed and constructed to direct a scanning laser at the photodetector array. By modulating the laser beam it is possible to selectively activate each photodetector to fire its associated nozzle. The laser scanner is programmed with raster data which defines the on/off pixel pattern of ink drops to be applied to the media. The use of a laser beam for transmitting the raster data eliminates the use of multiple interconnects typically formed by separate electrical conductors connecting each nozzle resistor of the printhead to the data processor. In a simple embodiment of the invention, the only electrical interconnect conductors required for the printhead are a power line and a ground line.
The advantages of the present invention can be obtained using laser-scanning technology that is well developed for electrophotographic printing systems. The PWA printhead construction uses known PWA construction, the difference being in the system for data transfer. Data is transferred to the PWA printhead by a laser scanning system similar to those used in electrophotographic systems. The laser scanner scans an array of photodetectors on the printhead. Each photodetector is associated with and electrically connected to a single firing resistor of an inkjet nozzle. Thus, an inkjet nozzle is actuated when the laser scanner is modulated to activate its associated photodetector.
There is no physical electrical connection between the printhead and the print data source for data transfer, as the data is now transferred by the scanning modulated laser beam. The data stream is much the same as a laser printer, where the data stream is used to create a raster image upon a photoelectric (i.e. photoconductive) surface. However, in the present invention, instead of creating an undeveloped electrostatic image, the scanner laser selectively activates individual photodetectors, which through activation of inkjets, results in selective creation of ink pixels on the media to form an image. As further described below, the data stream, and hence the modulations of the laser beam, may be identical to that used to modulate a laser in an electrophotographic system. However, the data stream may also be modified as desired to accommodate different designs for the PWA printhead and the photoconductor array.
Another advantage of the present invention is the mechanical simplicity. In addition to eliminating multiple conductors and connectors, the amount of moving parts is minimized. In one aspect of the invention, the only moving part for the pen or printhead is the scanning mirror for the laser scanner. Essentially, the only other moving parts are involved with the media-transfer system. In contrast laser printer systems require moving photosensitive belts or drums, and toner transfer and fixing systems, while inkjet printing requires carriage systems for the printhead with associated indexing and control systems.
The present invention can be easily adapted for either a monochrome printing system or a multicolor printing system. Color can be easily implemented using, for example, variations of multi-chamber inkjet designs known in the art.
The present invention can be seen as an optical multiplexing system where the data is transmitted to the PWA by an optical system, with the power to fire inkjet resistors carried to the PWA through electrical conductors. The only electrical connections required are for the power connection, since the data controlling the activation of the resistors is transmitted by the optical systems.